This application is a division of application Ser. No. 538,001 filed Sept. 30, 1983. This invention relates in general to color cathode ray having a flat faceplate and to a color selection electrode assembly for use therein. The invention is concerned with a method of manufacturing the electrode assembly and a cathode ray tube utilizing the assembly. The cathode ray tube which is the subject of the inventive method defined in this divisional application is described and claimed in the parent application.
In general, a color selection electrode or "shadow mask" is a device which is disposed adjacent the luminescent phosphor screen that forms the target electrode of a color cathode ray tube to control the landing pattern of one or more electron beams as they are swept across the screen. The shadow mask achieves color selection by partially shadowing the surface of the screen from the scanning electron beams, permitting access to selected elemental phosphor areas by those beams. The choice of a color selection electrode for use in color television cathode ray tubes is, by and large, a choice between a spherical or bi-radial electrode and a cylindrical electrode tensed upon a heavy spring frame--both types being supported within the tube envelope. The most common type of color selection electrode used in color television receivers today is the conventional curved type.
In color picture tubes utilizing a conventional shadow mask, there is a tendency on the part of the mask to "dome" (localized buckling) in those areas where a scene characterized by very high brightness is depicted. For example, in a scene where a high concentration of white is presented for an extended period of time, when the beams sweep that area of the screen, the current in each beam peaks precipitously with an attendant localized heating of the mask. As a result of such a concentration of heat, that area of the mask expands and displaces itself from its original "cold" position to a position in which it does not effect proper masking of the writing electron beams. As a result, color purity is degraded. Moreover, because of its vulnerability of "doming," a conventional mask cannot accommodate the power density that a doming-resistant tensed mask can.
The general practice in cathode ray tubes manufactured for use in color television receivers is to position the mask at an assigned location, relative to the phosphor screen, by suspending it from three preselected points disposed about the periphery of the tube's face panel. This suspension accommodates overall thermal expansion of the mask by causing the mask to be displaced toward the screen from its original position by provision of bimetallic support springs: however, such provision cannot resolve the above-described localized "doming" problem caused by concentrated heating in localized areas of the mask.
An advantage of utilizing a tensed mask resides in the fact that the mask, while under tension, will not dome. The mask retains its desired configuration under normal operating conditions.
The color television cathode ray tube in most common usage today employs a faceplate which approximates a section of a large radius sphere. The shadow mask in such a tube is contoured to match the faceplate. A trend today is toward a flatter faceplate which, in turn, calls for a flatter shadow mask. However, a flatter mask is inherently less mechanically stable than a more curved mask. Accordingly, to acquire mechanical stability, resort is made to a thicker mask, for example, one having a thickness in the order to 10 to 12 mils. This is approximately twice the thickness of a conventional curved mask. However, when one must utilize a 10 to 12 mil mask, the aperture etching process is much more difficult. Specifically, in order to prevent aperture limiting of the beam at the outer reaches of the mask, the apertures have to be etched at an angle to the plane of the mask, rather than etched more perpendicular to that plane as is the case for a conventional curved mask.
Four photographic exposures are required in a typical process for applying light-emitting phosphor deposits on the screen. Each of the exposures requires the projection of a light field from a light source, typically through a light-refracting lens and the shadow mask onto a photosensitve layer deposited on the inside surface of the viewing window. There is a separate exposure for each of the red, green and blue light emitting phosphor deposits; a fourth exposure may be required for depositing the grille, or black-surround prior to the phosphor exposure and deposition process. After each exposure, the face panel must be removed from the light projection system used for screening (a system commonly known as a "lighthouse") and subjected to a developing and wash cycle. Then the face panel is replaced in precise registration with the shadow mask and another exposure is made, with the light source displaced laterally for each exposure relative to the axis of the lens. This process of exposure, removal and replacement is continued until all three phosphor deposits and the grille deposit are completed.
The method requires a mechanism whereby the faceplate may be removed and replaced in exact, precise registration with the shadow mask. The allowable error in panel-to-mask registration is about 0.006 inch in the domed-type of shadow mask and about 0.0002 inch in foil-type tension-type shadow mask tubes.
The conventional shadow mask is "indexed"; that is, made repeatably registerable with the phosphor deposits screened on the faceplate. This type of mask is typically mounted in close adjacency to the faceplate by a suspension system comprising three or four leaf springs. The springs are welded to the mask frame at selected points around the periphery. The distal ends of the springs are apertured to engage studs which project inwardly from the rearwardly extending flange of the tube faceplate. In the screening process, in which the phosphors and grille are deposited, the mask-frame assembly must be capable of being demounted and remounted with exact precision in relation to the faceplate several times, as required in the manufacturing process. Demounting the mask is accomplished by depressing the springs to disengage the studs and separating the panel from the electrode, usually by automatic machinery. In remounting, following the deposition of a phosphor, the mask and faceplate are again brought into propinquity whereby the springs are caused to re-engage the studs. This process does not lend itself to the screening of a tube that utilizes the thin, foil mask heretofore described because of the lack of structural strength of the foil. The foil has no frame of the conventional type mountable by springs on the interior of the tube envelope.